Indeed, one of the main traits under selection, at least early in domestication, was stress coping behavior and reduced stress response. Domestic chickens possess lower stress responses in open field, aerial predator, novel object and fear of human tests demonstrated by lower frequencies of immobilization and freezing, and higher frequencies in vocalization, walking and flying. To characterize domestication at the genetic level several studies have used quantitative trait loci analysis and whole-genome resequencing and selective sweep analysis comparing the wild ancestor to domestic breeds. Selective sweep analysis is a method to identify positively selected gene variants important for domestication. A sweep region is characterized by loss of genetic variation indicating strong positive selection and is assumed to be associated with a mutation causing desirable traits. Among the domestication related selective sweeps identified by Rubin et al., one of the strongest ones is located adjacent to the coding region for aadrenergic receptor 2C. The sweep is 120 kb long on chromosome 4, and in a region void of known genes except for ADRA2C which is situated 10.8 kb outside of the region. Hence, ADRA2C stands out as the most likely d-Alaninol candidate causing the sweep. ADRA2C is predominantly present in the central nervous system and in the adrenals. ADRA2C works as an autoreceptor on presynaptic sympathetic neurons and it is involved in the regulation of norLycopene epinephrine release by a negative feedback loop. It is also the main regulator of epinephrine release from the adrenal medulla as shown in Adra2C knockout mice showing more than a 2-fold increase in circulating epinephrine. Behavioral alterations in Adra2c knockout mice are seen as an enhanced startle response, shortened attack latency and diminished acoustic prepulse inhibition. These findings were reversed in mice overexpressing Adra2C. For these reasons ADRA2C is tightly linked to the fight-and flight response and is a strong candidate gene to explain the effects of domestication on stress related behaviors.